Typically, injections are performed with syringes that pierce the skin with a needle to deliver medication to a desired location on a body. Piercing the skin in an area including or over high blood flows (otherwise know as free blood flows), such as from the presence of vascular bundles, arteries and/or veins, can cause the formation of subdermal hematomas (or bruising). These subdermal hematomas are unsightly, taking several weeks to heal, and are generally painful. If the needles are dull or have burrs, left over from manufacturing, this compounds the problem of creating subdermal hematomas.
As an alterative to needle delivery injections, needle-less medication injections have been performed with "permanent gun" instruments, generally referred to as "jet injectors". These devices use either a compression spring or a compressed inert gas to propel the fluid medication (via a push rod plunger) through a small orifice (an injector nozzle) which rests perpendicular to and against the injection site. The fluid medication is generally accelerated at a high rate to a speed of between about 800 feet per second (fps) and 1,200 fps (approximately 244 and 366 meters per second, respectively). This causes the fluid to pierce through the skin surface without the use of a needle, resulting in the medication being deposited in a flower pattern under the skin surface. This method of medication delivery is referred to as a subcutaneous injection. Again, piercing the skin in an area including or over high blood flows (such as from vascular bundles, arteries and veins) can cause the formation of subdermal hematomas (or bruising). Also with jet injectors, a used and worn delivery orifice can slow down the delivery speed of the injected fluid, which results in inadequate penetration and causes further subdermal hematomas (or bruising) of the skin at the injection site.
A drawback to all of these devices is that there is no way to determine if subdermal hematomas (or bruising) are likely to occur prior to administering an injection.